Transport layer fairness revisited

Fairness amongst the competing flows at the transport layer has always been an important topic, however, the current definition based on the TCP-compatible view is not always suitable. With the increasing deployment of multipath transport protocols such as Multipath TCP (MPTCP) and the Concurrent Multipath Transfer extension of SCTP (CMT-SCTP), the term “fair” can have various interpretations. In this paper, inconsistencies are avoided by classifying fairness definitions according to the resource - bottleneck or network - and the participants - subflow, flow, tariff, etc. that share the resource. With example network scenarios the current (TCP-compatible) fairness view from both the single and multipath perspective is presented and their shortcomings discussed. Alternative definitions are introduced and their benefits are illustrated based on a theoretical analysis. The realization aspects of the discussed fairness definitions are also presented. The evaluations of available coupled congestion control variants for multipath transport are shown to highlight the proximity of the simulated results to the theoretical target values. Due to the complexity of the realization of network-based approaches, bottleneck is chosen as the preferred resource. Tariff is a promising participant as it couples applications and incorporates economic entities for fair resource sharing at the transport layer¹.